The present invention relates to a method and a circuit for coherently recovering the phase and frequency of a received signal. More particularly, the invention relates to a method and a circuit for rapidly coherently recovering the phase and frequency of bursts of received signals from plural sources, which signals may differ from one another in frequency and phase.
Conventionally, coherent phase and frequency recovery has most often been performed with a PLL (Phase-Locked Loop) circuit, a block diagram of which is shown in FIG. 1. As shown in FIG. 1, the conventional PLL circuit is composed of a mixer 11, loop filter 12, and VCO (Voltage-Controlled Oscillator) 13 connected in a loop configuration. An input signal .theta. is applied to the positive input of the mixer 11, and the output of the mixer 11 is applied through the loop filter 12 to the frequency-control input terminal of the VCO 13. The output of the VCO is connected back to the negative input of the mixer 11. As is well known, when a new input signal .theta. is applied to this circuit, if the frequency and/or phase of the input signal .theta. differ from those of the output of the VCO 13 (and they are within the pull range of the circuit), the resulting output of the loop filter 12 will drive the VCO 13 in the direction to cause the output of the VCO to follow the input signal .theta. in frequency and phase.
The PLL circuit of FIG. 2 can be modeled by the nonlinear phase parameter control system shown in FIG. 2. In this model, the VCO is represented as an integrator having a transfer function K.sub.0 /s, and the mixer 11 by an adder 14 and a block having a transfer function K.sub.d .multidot.sin(s). The loop filter has a transfer function F(s).
To achieve phase lock with an incoming signal of arbitrary phase, the output phase estimate .theta. from the integrator must change to approximate the value of the phase of the input signal .theta.. When a new input signal .theta. is applied, it is represented in the model of FIG. 2 by a step change in input phase. For the VCO to instantaneously follow a step change, it must be driven by an impulse, which requires a very large bandwidth. This results in two fundamental and inherent problems.
First, for applications that employ a crystal-controlled VCO with a relatively narrow pull range, it is generally difficult, if not impossible, to prevent the VCO input from saturating at wide recovery loop bandwidths. In addition, if the input SNR (Signal-to-Noise Ratio) is not sufficiently large, the wider bandwidth will yield a poorer acquisition reliability. That is, the probability of missing acquisition and falsely detecting acquisition are limited by what is termed the "hangup" phenomenon. In general, the lower the SNR, the greater the chance hangup will occur.
Consequently, an alternate circuit arrangement that eliminates the PLL so as to avoid the hangup phenomenon while maintaining a moderate acquisition bandwidth is often needed. For this purpose, a tuned filter is sometimes used. A tuned filter, however, has the disadvantage that if there is a frequency offset in the incoming signal, a phase error in the output estimate will result.